At a Liquefied Natural Gas (LNG) plant, a new LNG bunkering line is being constructed to supply ships with liquid LNG. This project involves the installation of new piping, which will be completed in multiple phases. This case study focuses on evaluating the material stresses, as well as the loads on restraints, nozzles, and flanges, in accordance with the relevant engineering codes.
Analysis
The subject of the analysis is a 10-inch suction line connected to a cryogenic pump. This suction line includes a section of Vacuum Insulated Piping (VIP), a bypass to the discharge piping, and standard insulated piping. Since the discharge piping is linked to the suction piping via a bypass, it was essential to model the discharge piping accurately to determine the stresses in the suction line.
The stresses in the discharge piping were not evaluated in this study. The analysis incorporated the applicable pump loads, which were assessed using data obtained from a flexibility study.
A key characteristic of this model is the significant displacements observed at the header to which the suction line connects. These displacements are attributed to the operating temperature of -160°C. Due to these substantial displacements, it is crucial to ensure that the stresses at the tie-in are minimized by allowing sufficient flexibility in the connecting piping. Several iteration loops were conducted to determine an optimal leg length, ensuring that the stresses remained within acceptable limits.
Key Findings
Static Stress Analysis: A static stress analysis was conducted for an 8-inch bunkering line and a portion of a 12-inch LNG header. The static stresses were found to be acceptable according to the allowable values specified by the ASME B31.3 code for pressure piping.
Nozzle Loads: Initial evaluations indicated that the nozzle loads exceeded the allowable limits. However, subsequent calculations performed by the client demonstrated that the nozzle loads are within acceptable limits.
Dynamic Stress Analysis: A dynamic stress analysis was performed for the main line, and the results were assessed in accordance with the ASME B31.3 code for pressure piping. The dynamic loads were based on a review of the surge report for the original system. The dynamic stresses were found to be within the allowable limits specified by the ASME B31.3 code.
Conclusion
The analysis of the LNG bunkering line confirms that both static and dynamic stresses are within acceptable limits as per the ASME B31.3 code for pressure piping. The study highlights the importance of accommodating large temperature variations to manage displacements and ensure the structural integrity of the piping system. The findings demonstrate that with appropriate design considerations and flexibility, the stresses in the new LNG bunkering line remain within the allowable thresholds.
